A car seat is generally equipped with a seat reclining device by which an angular position of a seatback with respect to a seat cushion can be varied. One such vehicle seat reclining device has been disclosed in Japanese Patent Provisional Publication No. 2008-212176 (hereinafter is referred to as “JP2008-212176”), corresponding to United States Patent Application Publication No. US 2008/0203799 A1, published Aug. 28, 2008 and assigned to the assignee of the present invention. The seat reclining device, disclosed in JP2008-212176, includes a pair of base brackets fixedly connected to respective sides of a seat cushion, and a pair of arm brackets fixedly connected to respective sides of a seatback. A base-side disk-shaped case is fixedly connected to the associated base bracket, whereas an arm-side disk-shaped case is fixedly connected to the associated arm bracket. The base-side case and the arm-side case are assembled to each other by means of a ring-shaped holder in such a manner as to restrict axial movement of these cases and to permit rotary motion of the arm-side case relative to the base-side case. A pair of lock tooth blocks are installed in the base-side case in such a manner that the lock tooth blocks can pivot about respective tooth-block supporting portions (respective tooth-block pivots). A [spring-loaded] cam member, which is permanently biased by means of a pair of lock springs, acts to force the lock tooth blocks toward their locked positions. By virtue of rotary motion of the [spring-loaded] cam in one rotational direction, the external toothed portion of each of the lock tooth blocks is brought into meshed-engagement with the internal toothed portion formed on the inner periphery of the arm-side case. With the external and internal toothed portions meshed with each other, the arm-side case is kept in its locked state with respect to the base-side case. Conversely when the cam member is rotated in the opposite rotational direction against the spring force of the lock springs, the external toothed portion of each of the lock tooth blocks is de-meshed from the internal toothed portion of the arm-side case. With the external and internal toothed portions de-meshed from each other, the arm-side case is in its unlocked state with respect to the base-side case. A pair of axially-protruded tooth-block receiving portions are provided in close proximity to both circumferential ends of each of the lock tooth blocks, so as to function as a tooth-block guide, permitting oscillating motion of the associated lock tooth block, and also to function to prevent relative rotation of the arm-side case to the base-side case in the two opposite rotational directions by the lock tooth blocks whose external toothed portions are in meshed-engagement with the respective arm-side case internal toothed portions. On the one hand, formed on the inside face of the base-side case are the tooth-block supporting portion pair (the tooth-block pivot pair), the two tooth-block receiving portion pairs (the two tooth-block guide pairs), and a pair of lock-spring supporting protrusions to which the innermost end portions of the respective lock springs are hooked. On the other hand, formed on the outside face of the base-side case are circumferentially-spaced four circular attachment protrusions, which protrusions are inserted into respective attachment holes of the base bracket when assembling, and a pair of circumferentially-spaced diametrically-opposed protrusions (hereinafter called “spring accommodation protrusions”) which protrude from the outside face of the base-side case and by which a pair of diametrically-opposed spring accommodation recesses are necessarily defined on the inner peripheral wall surface of the base-side case. The previously-discussed plural protrusions, each formed on either the inside face or the outside face of the base-side case, are formed or shaped by pressing (exactly, half-blanking). In pressing protrusions and/or recesses in the same base-side case, it is difficult that one (e.g., the lock-spring supporting protrusion onto which the innermost end portion of the lock spring is hooked) of the two different protrusions, protruding in the two opposite directions, is formed within the same area of the base-side case in a manner so as to be superimposed on the other protrusion (e.g., the spring accommodation protrusion protruding from the outside face of the base-side case). Taking account of the strengths of the two cases, for instance, the mechanical strength of the base-side case, increasing a mechanical strength of the tooth-block supporting portion (the tooth-block pivot) usually requires a trade-off in mechanical strength of the attachment protrusions (hereinafter referred to as “anti-rotation protrusions”) serving to prevent undesirable rotary motion of the base-side case relative to the base bracket. In recent years, it would be desirable to provide a seat reclining device configured to balance two contradictory requirements, namely, an increased mechanical strength from the viewpoint of safety and a more compact installation from the viewpoint of increased seat-layout flexibility. In the case of the seat reclining device, disclosed in JP2008-212176, for the same area as the spring accommodation protrusion, protruding from the outside face of the base-side case, in other words, the spring accommodation recess necessarily formed on the opposite side (i.e., the backface) of the spring accommodation protrusion, the lock-spring supporting protrusion, protruding from the inside face of the base-side case, is formed substantially at a center of the spring accommodation recess. As set forth above, the lock spring is installed in this spring accommodation recess, and thus there is a drawback of the greatly-limited shape in lateral cross section of the lock-spring supporting protrusion.
To avoid this, Japanese Patent Provisional Publication No. 8-253063 (hereinafter is referred to as “JP8-253063”), corresponding to U.S. Pat. No. 5,681,086, teaches a different spring supporting structure for a lock spring. In the vehicle seat articulation disclosed in JP8-253063, three coiled torsion springs are provided so as to force three lock tooth blocks radially outwards, that is, toward their locked positions (meshed-engagement positions) via a central cam member. One end (a straight short hook) of each of the torsion springs is kept in abutted-engagement with one of the two opposing circumferential end faces of one of two circumferentially adjacent boss pairs, each boss pair corresponding to tooth-block guides. The other end (a radially-inward two-step bending hook) of the torsion spring is kept in engagement with the cam member. The outer circumference of the winding portion of the torsion spring is supported by the two circumferentially adjacent bosses, such that the winding portion of the torsion spring is partly sandwiched between the two adjacent bosses. Thus, the spring supporting structure as disclosed in JP8-253063 eliminates the necessity of the lock-spring supporting protrusion as disclosed in JP2008-212176.
However, each of the three boss pairs is formed to protrude from the inside face of a fixed metal flange (corresponding to the base-side case), which flange is integral with the framework of the seat part, and arranged close to the circumference of the fixed metal flange. Also formed to protrude from the outside face of the fixed metal flange are circumferentially-spaced attachment protrusions (i.e., “anti-rotation protrusions”), which protrusions are inserted into respective attachment holes of a base bracket (to which the fixed metal flange is attached) to prevent undesirable rotary motion of the fixed metal flange relative to the base bracket. In order to enhance a mechanical strength, concretely, to increase a moment created by the fitting portions of the attachment protrusions (“anti-rotation protrusions”) of the fixed metal flange and the attachment holes of the base bracket, which moment counteracts or prevents undesirable rotary motion of the fixed metal flange relative to the base bracket, it is desirable to arrange the attachment protrusions (“anti-rotation protrusions”) in close proximity to the circumference of the fixed metal flange, as much as possible. However, this leads to another problem of overlapping of the boss, protruding from the inside face of the fixed metal flange, with the attachment protrusions (“anti-rotation protrusions”), protruding from the outside face of the fixed metal flange. Hence, in the vehicle seat articulation disclosed in JP8-253063, the attachment protrusions (“anti-rotation protrusions”), protruding from the outside face of the fixed metal flange, are arranged to be offset radially inwards from the boss, protruding from the inside face of the fixed metal flange. This radially-inward layout of the attachment protrusions (“anti-rotation protrusions”) is inferior in increased mechanical strength (i.e., increased counteracting moment).
Thus, in order to balance an increased mechanical strength and an increased seat-layout flexibility, it would be desirable to provide a vehicle seat reclining device having a more improved spring supporting structure.